1. Field of the Invention
The present invention relates to an elliptically polarizing plate and a liquid crystal display (LCD), which maintains a sufficient display contrast in all azimuths and which has a display performance that does not generate a color shift.
2. Description of the Prior Art
Currently, a mainstream method of a general liquid crystal display is a thin-film transistor (TFT) LCD using a twisted nematic (TN) liquid crystal. In this method, there is available an advantage in that a response speed is high, and a high contrast can be obtained. However, when a display panel using a TN liquid crystal is seen from a tilt angle to direction of a normal line, the contrast is remarkably lowered, and a gray scale reversion is generated in which the gray scale display is reversed. The display panel has a characteristic such that a viewing angle is extremely narrow. In the usage of a large-scale monitor and a television or the like, a high contrast, a wide viewing angle, and a small change in the display color number resulting from a viewing angle are demanded. Consequently, in the case the TN mode TFT-LCD is used in such usage, a retardation film for compensating for the viewing angle is indispensable.
As a retardation film, a stretched birefrigent polymer film has been conventionally used. Recently, it is proposed that a compensation sheet having a compensation anisotropy layer formed of liquid crystal molecule on a transparent substrate be used, instead of the compensation sheet comprising the stretched birefrigent film. Since there are various alignment forms in the liquid crystal molecule, it becomes possible to realize optical properties, which cannot be obtained with the conventional stretched birefrigent polymer film, by using the liquid crystal molecule.
As the above retardation film for the viewing angle compensation film, for example, there is available a wide view film (trade name WV film) manufactured by Fuji Film Co., Ltd. which uses a discotic liquid crystal having negative anisotropy. An object of this retardation film is to improve viewing angle properties in the state of black display, namely, in the voltage application state. That is, in the voltage application state, the liquid crystal molecule in the liquid crystal cell shows positive refractive index anisotropy having an optical axis, which is tilted from the glass substrate. The retardation film comprising the liquid crystal molecule having an optical axis tilted from a normal line and negative refractive index anisotropy to compensate the retardation of the refractive index anisotropy.
Furthermore, as another retardation film for viewing angle compensation, there is available a HN film manufactured by NISSEKI MITSUBISHI Co., Ltd. which uses a rod-like nematic liquid crystal having a positive refractive index anisotropy. An object of this retardation film is to primarily compensate for an intermediate gray scale display state and the film comprises a liquid crystal molecule having an optical axis tilted from the normal line direction of the film and positive refractive index anisotropy.
Furthermore, Japanese Unexamined Patent Publication No. HEI 7-306406, Japanese Unexamined Patent Publication No. HEI 7-35924 and Japanese Unexamined Patent Publication No. HEI 10-123506 describe a retardation film laminating a compensation layer having an optical axis tilted from the film normal line and compensation layer having positive uniaxial properties and an optical axis within the film plane, so that each main refractive index directions orthgonally, using a rod-like nematic liquid crystal in order to compensate for a viewing angle of a display contrast and a gray scale display state.
Also, Japanese Unexamined Patent Publication No. HEI 2000-304930 describes that the liquid crystal cell can be accurately compensated by using together a discotic liquid crystal and a rod-like liquid crystal, which have completely different optical properties. As to the state of the optical anisotropic layer with respect to the TN mode in particular, it is described that the discotic liquid crystal molecules are tilt aligned, then the rod-like liquid crystal can be optically accurately compensated when the average tilt angle is set to less than 5°.
Above described, it becomes possible to optically compensate for the liquid crystal cell to some degree by using a compensation sheet in which the liquid crystal molecules are tilt aligned on the transparent substrate. However, in the case where only such optical compensation films are used, the display contrast is remarkably lowered when these sheets are tilted at approximately 60° or more from the panel normal line. In particular, in the case where the rod-like nematic liquid crystal molecule is used, the optical compensation is conducted with the liquid crystal molecule having the same optical anisotropy as the molecule used in the liquid crystal cell in the TN mode. As a consequence, it is theoretically inappropriate to compensate for the orientation state in which an optical axis of the rod-like nematic liquid crystal cell has a tilt approximate to the normal line direction with respect to the cell surface in the black state. When the viewing angle is changed, the black sink cannot be maintained so that a bluish black is provided. Furthermore, in the case where the discotic liquid crystal molecule is used, colorization to the yellow color can be observed when the viewing angle is changed in the white state, namely in the case of no application of voltage.
Furthermore, a film combination of the tilt orientation layer of the rod-like (i.e., “calamitic”) nematic liquid crystal molecule and the retardation layer having positive uniaxial properties is known. With respect to the film, it has been made clear as a result of the inspection of these reports that a conspicuous colorization is generated by a change in the viewing angle. Thus, only a practically unfavorable liquid crystal can be obtained.
Furthermore, in the case where the discotic liquid crystal and the rod-like nematic liquid crystal are used together, the viewing angle properties of the contrast are widened when the average tilt angle is less than 5°. However, the colorization phenomenon is conspicuous when the viewing angle in the white display is changed so that the problem of the deterioration in the display quality cannot be solved.
In this manner, as the retardation film used in the viewing angle compensation of the TN mode TFT-LCD, a birefrigent retardation film is used in which a tilt orientation layer of the discotic or rod-like nematic liquid crystal layer is formed on the transparent substrate. However, no retardation film is observed in which the total viewing angle properties such as a display contrast and the colorization are not remarkably improved with accurate retardation compensation.
An object of the present invention is to provide an elliptically polarizing plate, with accurate retardation compensation of the liquid crystal cell, for a liquid crystal display that maintains a display contrast having a sufficient visibility when the viewing angle is changed and which does not generate colorization.
And an object of the present invention is to provide an A liquid crystal display using the above-mentioned elliptically polarizing plate.